This application is based on Application No. 10-297928 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an image taking optical system for cameras, and specifically relates to an image taking optical system having a reduction optical system for reducing an image via a main optical system.
2. Description of Related Art
In conjunction with the popularization of personal computers in recent years, digital still cameras for taking images which are easily stored on floppy disks and the like have become widely used. With the increasing popularity of digital still cameras, there has been demand for enlargement of the photographic range, and concomitant demand for photographic lenses of various specifications.
In the field of recording images on silver salt film, single lens reflex cameras have come to use a plurality of exchangeable taking lenses (exchangeable lenses), and there has been a corresponding demand for taking lenses of various specifications.
If exchangeable lenses for single lens reflex cameras can be used with digital still cameras using an photoelectric conversion element, it is possible to respond to the demand for such photographic lenses. A specific method, for example, arranges a direct photoelectric conversion element on the image forming plane of an exchangeable lens. Japanese Laid-Open Patent Application Nos. 63-205626, 7-253537, and 8-114742 disclose methods wherein a condenser lens is arranged near the image plane of an exchangeable lens, and a relay optical system is provided for re-forming an image formed by the exchangeable lens.
On one hand, in such methods which arrange a direct photoelectric conversion element on the image forming plane of an exchangeable lens, an extremely large photoelectric conversion element having a large number of pixels is required to obtain the full optical capabilities of the exchangeable lens. This is because the conversion element has a screen size similar to that of silver salt film. Such an photoelectric conversion element is prohibitively expensive and difficult to apply to consumer products.
On the other hand, the constructions disclosed in Japanese Laid-Open Patent Application Nos. 63-205626, 7-253537, and 8-114742 are disadvantageous inasmuch as the taking optical system is greatly enlarged so as to re-form the image formed by the exchangeable lens.
An object of the present invention is to provide an improved image taking optical system for cameras.
Another object of the present invention is to provide a compact taking optical system for cameras capable of using exchangeable lenses, and which is suitable for producing high quality images.
These objects are attained by an image taking optical system comprising a main optical system for forming an object image, and a reduction optical system for reducing an image without re-forming the image formed by the main optical system, wherein the lens having the strongest positive optical power within the reduction optical system satisfies the following conditions:
xe2x88x922.5  less than (rP1xe2x88x92rP2)/(rP1+rP2) less than xe2x88x920.3, and
1.2 less than xcfx86P/xcfx86s less than 5.0,
where rP1 represents the radius of curvature on the object side of the positive lens, rP2 represents the radius of curvature on the image side of the positive lens, xcfx86P represents the optical power of the positive lens, and xcfx86s represents the optical power of the total system of the reduction optical system.